A girl's first period, called menarche (say: MEH-nar-kee), signals that she is growing up and her body is preparing so that she might have a baby someday. A period is the 2 to 7 days that a girl or woman experiences menstrual flow — blood and tissue that leaves the body through the vagina.

For most girls, it amounts to about 2 tablespoons (30 milliliters). To catch the blood and prevent stains on her clothes, a girl can wear a pad, which sticks to her underwear, or a tampon, which is inserted into the vagina.

What's Going On Inside?

The blood and tissue leaves the body because it's no longer needed. It comes from the uterus, the organ inside a woman's body where a baby grows. Each month, blood and tissue build up in the uterus in case the woman becomes pregnant. That lining would be needed if the woman's egg was fertilized by a man's sperm cell. A fertilized egg attaches to that cushiony lining and begins growing into a baby.

But most of the time, the egg does not get fertilized, the lining is shed, and the girl or woman has her period.

Let's talk for a minute about eggs. They're not the kind you buy in a carton at the store! Girls and women have two ovaries. Each of these ovaries holds thousands of eggs, which are tiny (each no bigger than the tip of a pin). During the menstrual cycle, an egg is released from one of the ovaries and begins a trip down one of the fallopian (say: fuh-LO-pee-un) tubes to the uterus, also called the womb.

If a sperm cell does not fertilize the egg, the unfertilized egg and the lining from the uterus leave the body. In other words, a girl has her period. The cycle then begins again. The lining of the uterus will start building up, and about 2 weeks after the last period, another egg will be released.

What's a Cycle?

When people talk about the menstrual (say: MEN-strul) cycle, they usually mean the days when blood and tissue (menstrual fluid) leaves the body through the vagina. That's the most visible part of the process, and the part that girls and women need to manage.

But the monthly cycle is exactly that — a process that takes about a month. A normal menstrual cycle for girls and teens ranges from 21 to 45 days. At most, only a week of the cycle involves menstrual fluid exiting the body. A normal period lasts from 2 to 7 days.

The rest of the time, the girl doesn't have any bleeding but other stuff is happening, like the lining building up and the egg being released.

When Periods Start

Most girls start to menstruate between ages 10 and 15 years. The average age is 12, but every girl's body has its own schedule.

Although there's no one right age for a girl to get her period, there are some clues that menstruation will start soon. Typically, a girl gets her period about 2 years after her breasts start to develop. Another sign is vaginal discharge fluid (sort of like mucus) that a girl might see or feel on her underwear. This discharge usually begins about 6 months to a year before a girl gets her first period.

What's Normal?

Especially when menstrual periods are new to a girl, it can be tough to know what's normal and what's not. Talking to your mom or another grownup woman is a good idea since they've been through it.

But for some problems, it's best to talk with the doctor. Here are some of them:

Your period lasts longer than a week.

You have to change your pad very often (soaking more than one pad every 1-2 hours).

You go longer than 3 months between periods.

You have bleeding in between periods.

You have an unusual amount of pain before or during your period.

Your periods were regular but became irregular.

Here are some additional concerns, which many girls have when they are just starting their periods.

What If My Periods Are Irregular?

For older girls and women, their periods pretty much stick to a regular pattern. The entire cycle lasts the same number of days (21 to 34) and they have bleeding for about the same number of days (less than a week). For the first year, a girl's periods are often irregular and hard to predict.

It's a good idea to keep track of your period with a calendar. Talk with your doctor if you're concerned about the length of your period or cycle. About 6 years after a girl's first period, the cycles usually get shorter and more regular.

What If I Have Cramps?

For minor pain, you can take over-the-counter medications like ibuprofen. Other things you can do that may help with menstrual cramps include:

regular exercise

heating pads on your lower abdomen

meditation or relaxation

Talk to your doctor if these solutions don't work for you, especially if your period is keeping you from going to school or participating in stuff you want to do, like activities and sports.

How Much Bleeding Is Too Much?

Menstrual flow can seem like a lot, but usually isn't. Most girls change their pad about three to six times a day over the course of a period — with more changes when the flow is heaviest and fewer changes when the flow is lighter — usually at the very beginning and end.

Let your doctor know if your flow seems heavier than this or lasts more than a week.

What If I Get Moody Around My Period?

Hormone changes can cause girls to feel more sad or irritable before their periods start. Talk to your doctor if this happens to you. Being physically active is a natural mood lifter, so regular exercise often helps.

In Praise of Periods!

It's normal to be a little nervous about your first period. And if you've already had your period a while, it's OK to dislike the inconveniences it can cause. But don't forget to feel proud, too. Menstruation is a sign of normal growth and development. In other words, you're healthy and you're growing up just the way you should!